Non-metal inserts for bone support assembly

ABSTRACT

A bone support structure is formed with opposing dynamization windows, and spacers of a bioresorbable material are positioned within the dynamization windows. The dynamization windows are longer than they are wide. The spacers may be integrally formed as a single insert. The bone support assembly is used with a bone fastener such as a bone screw which is advanced transversely through both the insert and the bone. The bone fastener is smaller across than the dynamization windows, so each spacer spaces the bone fastener relative to its dynamization window. As the spacers resorb, stress (at least in one direction) is increasingly transmitted through the fracture site rather than through the bone support structure. The positioning of the bone fastener, the shape and size of the dynamization windows and spacers, and the material of the spacers all allow design control over the type and amount of dynamization seen at the fracture site. Also, because the bone fastener is smaller across than the dynamization windows and spacers, a larger error in placement of the bone fastener is permissible. The insert can be selected by the surgeon and placed into the bone support structure based upon desired treatment modalities.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a Continuation-in-Part of U.S. application Ser. No.09/575,764, filed May 22, 2000 now U.S. Pat. No. 6,709,436 and entitledNON-METAL SPACERS FOR INTRAMEDULLARY NAIL, which is a continuation ofPCT application no. PCT/US00/09582, filed Apr. 10, 2000, which is aContinuation-in-Part of U.S. application Ser. No. 09/289,324, filed Apr.9, 1999, entitled INTRAMEDULLARY NAIL WITH NON-METAL SPACERS, now issuedas U.S. Pat. No. 6,296,645.

BACKGROUND OF THE INVENTION

The present invention relates to intramedullary nails used for treatmentof a fracture of a bone having a medullary canal extendinglongitudinally within the bone, and particularly to the structure of theintramedullary nail and methods for anchoring the intramedullary nailwith respect to one or more fragments of the fractured bone. The presentinvention also relates to bone plates used for treatment of a fractureof a bone placed upon the surface of one or more fragments of thefractured bone, and to similar bone support structures which are used toanchor or support two portions of bone relative to one another.

Intramedullary nails are used by orthopedic surgeons to treat fracturesinvolving long bones such as the femur, humerus, tibia, fibula, etc. Themedullary canal of the fractured bone is drilled out or otherwise openedfrom one end, and the intramedullary nail is longitudinally placedwithin the medullary canal to contact at least two fragments, i.e., suchthat the nail extends on both sides of the fracture. As used herein, theterm “fragment” refers to a portion of a fractured bone regardless ofwhether the fracture is complete. When implanted, the nail strengthensand supports fragments of the fractured bone during healing of thefracture.

Various types of intramedullary nails are well known within the medicaldevice arts, and several different methods have been used to attach theintramedullary nail within the bone. For instance, in U.S. Pat. No.4,338,926 to Kummer et al., an intramedullary nail is disclosed whichplaces a compressive force radially outward on the interior wall of thecortex structure surrounding the intramedullary nail. The compressiveforce secures the Kummer nail within the medullary canal of thefragments. Similarly, in U.S. Pat. No. 4,457,301 to Walker a flexibleplastic core elements holds longitudinal pins of an intramedullary nailin place. In U.S. Pat. No. 5,514,137 to Coutts, cement is injectedthrough a cannula in an intramedullary nail to secure the distal end ofthe intramedullary nail to the bone. Other intramedullary nail designsemploy a more secure and mechanically positive attachment to the bone,such as through use of one or more bone fasteners which extendtransversely to the longitudinal axis of the nail and through the cortexof the bone. The bone fastener is received within a receiving recess orthrough-hole within the intramedullary nail to secure the intramedullarynail relative to the bone fastener. In the transverse attachment, thereceiving opening defines an axis which is at an angle to thelongitudinal axis of the nail (90° and 45° angles are common), and thebone fastener is advanced on this receiving opening axis. U.S. Pat. No.4,733,654 to Marino, U.S. Pat. No. 5,057,110 to Kranz et al., U.S. Pat.No. 5,127,913 to Thomas, Jr., U.S. Pat. No. 5,514,137 to Coutts(proximal end) and others disclose such a transverse bone fastenerattachment in a bicortical attachment. U.S. Pat. No. 5,484,438 to Pennigshows a nail design with a recess which permits only unicorticalattachment. The present invention particularly relates to intramedullarynails which use bone fasteners transversely through the cortex forattachment.

Bone plates are used by orthopedic surgeons to treat many types ofbones, to support two portions of bone relative to one another. The boneplate is positioned to extend from one portion of bone to the otherportion of bone, typically in direct contact with both bone portions.For instance, the bone plate may be positioned such that the plateextends on both sides of a fracture. The bone plate is separatelysecured to each of the supported bone portions, such as by bone screws.When implanted, the bone plate supports tension, compression and/orbending stresses from one portion of bone to the other.

Problems may arise when attaching an intramedullary nail or bone plateto a fragment or other bone portion with a bone fastener. It isoccasionally difficult for the surgeon to properly align the bonefastener and/or a hole for the bone fastener with the receiving openingon the nail or plate. Part of the error is unique to intramedullarynails, due to difficulty in aligning the bone fastener with thereceiving opening when the receiving opening is within the bone.Additionally, the nail may be slightly bent during insertion of the nailstructure into the medullary canal. Such bending of the nail structuremay be desired in some instances so the nail shape better matches theparticular shape of the medullary canal for a particular patient.Regardless of whether intended or unintended, bending of the nailstructure creates further alignment errors between the bone fastenerand/or a hole for the bone fastener and the receiving opening on thenail. Other alignment difficulties are common to both bone plates andintramedullary nails. For example, other bones, bony growths, oroverlying tissue may make placement of the intramedullary nail or boneplate and insertion of the bone screws difficult. Four types ofalignment errors can be identified: (a) in transverse displacement(e.g., when the axis of the bone fastener is in the same transverseplane as the receiving opening in the nail/plate but does not intersectthe longitudinal central axis of the nail/plate), (b) in longitudinaldisplacement (i.e., when the bone fastener is at a differentlongitudinal location than the receiving opening in the nail/plate), (c)in longitudinal angular misaligned (i.e., when the axis of the receivingopening and the axis of the bone fastener are at different anglesrelative to the longitudinal axis of the nail/plate), and (d) intransverse angular misaligned (i.e., when the axis of the receivingopening and the axis of the bone fastener are in the same transverseplane but at different radial positions relative to the nail/plate).

Various types of jigs have been proposed to reduce alignment errors,such as shown in U.S. Pat. No. 4,733,654 to Marino and U.S. Pat. No.5,776,194 to Mikol et al. Primarily used with intramedullary nails, thejig may be temporarily attached to the proximal end of the nail to helpalign the bone fastener and/or the drill to the receiving opening in thenail. While such jigs are helpful, they become less reliable as distancefrom the proximal end of the nail increases, particularly if any bendingof the intramedullary nail has occurred. Though less commonly used withbone plates, jigs can be used to position a drill and/or a bone fastenerrelative to the holes in the bone plate. Additional solutions areneeded, especially for attaching the distal end of the intramedullarynail to a distal fragment.

A second method to reduce such alignment problems is to locate thereceiving openings in-situ, such as through an x-ray or through the useof magnets as taught in U.S. Pat. No. 5,127,913 to Thomas, Jr. Suchmethods are not typically preferred by surgeons in as much as theyrequire significant additional time and effort during the orthopedicsurgery, to the detriment of the patient.

A third method to reduce such alignment problems is to drill thereceiving opening into the bone plate or intramedullary nail only afterthe plate or nail is positioned relative to the bone, allowing thereceiving opening to be formed at a range of locations. Such in-situdrilling is taught in U.S. Pat. No. 5,057,110 to Kranz et al., wherein atip section of the intramedullary nail is formed of a bioresorbablematerial. However, bioresorbable materials are not as strong as metals,ceramics or other materials, leading to a product which is weaker thandesired and has a weaker attachment than desired.

Further problems occur during placement of the bone plate orintramedullary nail. For minimal damage to cortical tissue of the boneand most beneficial healing using an intramedullary nail, both the holethat is drilled in the medullary canal for the nail and then the nailitself need to be precisely located and secured with respect to themedullary canal. For bone plates, the surface contour of the bone orother tissue may not allow the bone plate to be positioned exactly asdesired, such as bone surface contours which do not allow the bone plateto be in full contact with the bone throughout the length of the boneplate.

Additional problems with bone support structures occur due to thehealing requirements of the bone with respect to the strength andrigidity of the nail/plate. U.S. Pat. No. 4,756,307 to Crowninshield andU.S. Pat. No. 4,338,926 to Kummer et al. disclose intramedullary nailswith bioresorbable portions to weaken the nail relative to the bone overtime, but these nails forsake the use of a transverse bone fastener toachieve this benefit.

U.S. Pat. No. 5,935,127 to Border and French Patent Publication No.2,710,835 to Medinov disclose at least partially filling the opening ina bone support device with an amount of resorbable material. However,both of these devices appear to be fully assembled during manufacture.That is, neither of these devices leave the surgeon with flexibility inwhether and when to place the the resorbable material into the opening,or in any selection as to the type of resorbable material to be used.Also, both the Border patent and the Medinov patent fail to consider howto adequately and optimally secure the resorbable material to the bonesupport device.

BRIEF SUMMARY OF THE INVENTION

The present invention involves a bone support implant to treatment of abone. The bone support implant is formed with at least one window in anexterior side, and an insert of a non-metal material is selected andpositioned within the window. The bone support implant is used with abone fastener such as a bone screw which is advanced transverselythrough the insert in the implant and through the bone. In one aspect ofthe invention, the non-metal insert is formed of a bioresorbablematerial, and the window is a dynamization window. As the bioresorbableinsert resorbs, stress is increasingly transmitted through the fracturesite rather than through the bone support implant. The positioning ofthe bone fastener, the shape and size of the window and insert, and thematerial of the insert all allow control over the type and amount ofdynamization seen at the fracture site. Use of a separate insert, whichis placed into the implant structure by a treating physician, allowsselection of a non-metal insert which has appropriate features and/orhad been appropriately treated and handled to best match the desiredhealing modality of the particular fracture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of an intramedullary nail in accordancewith the present invention.

FIG. 2 is a cross-sectional view taken along lines 2—2 in FIG. 1.

FIG. 3 is a cross-sectional view taken along lines 3—3 in FIG. 2.

FIG. 4 is a cross-sectional view taken along lines 4—4 in FIGS. 1 and 3.

FIG. 5 is a cross-sectional view taken along lines 5—5 in FIGS. 1 and 3.

FIG. 6 is a cross-sectional view taken along lines 6—6 in FIGS. 1 and 3.

FIG. 7 is a plan view of the insert used in FIGS. 1-6.

FIG. 8 is an elevational view of a first alternative insert for use withthe nail structure of FIGS. 1-6.

FIG. 9 is a plan view of the first alternative insert of FIG. 8.

FIG. 10 is an elevational view of a snap fit alternative insert.

FIG. 11 is a cross-sectional plan view of the snap fit alternativeinsert of FIG. 10, shown during insertion into an alternative nailstructure.

FIG. 12 is a perspective view in partial cross-section showing use ofthe snap fit alternative insert of FIGS. 10 and 11 as used in a boneplate.

FIG. 13 is an exploded perspective view depicting packaging of theinsert of FIGS. 1-7 into a preferred sealed container.

FIG. 14 is a plan view of third alternative insert used in acorresponding nail structure.

FIG. 15 is an exploded plan view of a fourth alternative insert foraxial insertion into a corresponding nail structure.

FIG. 16 is a cross-sectional view taken along line 15—15 in FIG. 14after assembly.

FIG. 17 is a cross-sectional view of a fifth alternative insert axiallyinserted into a corresponding nail structure.

FIG. 18 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a first type of attachment to a bone.

FIG. 19 is a cross-sectional view taken along lines 18—18 in FIG. 17.

FIG. 20 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a second type of attachment to a bone.

FIG. 21 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a third type of attachment to a bone.

FIG. 22 is a cross-sectional view of a distal end of the nail of FIGS.1-6 in a fourth type of attachment to a bone.

FIG. 23 is a cross-sectional view similar to FIG. 19 showing a firsttype of permissible offset.

FIG. 24 is a cross-sectional view similar to FIG. 19 showing a secondtype of permissible offset.

While the above-identified drawing figures set forth one or morepreferred embodiments, other embodiments of the present invention arealso contemplated, some of which are noted in the discussion. In allcases, this disclosure presents the illustrated embodiments of thepresent invention by way of representation and not limitation. Numerousother minor modifications and embodiments can be devised by thoseskilled in the art which fall within the scope and spirit of theprinciples of this invention.

DETAILED DESCRIPTION

An intramedullary nail 20 according to the present invention includes anail structure 22 with a proximal end 24, a distal end 26 and a shaft28, with “proximal” and “distal” being defined in accordance with thedirection the nail 20 is intended to be inserted into the bone. As knownin the art, the dimensions of the proximal end 24, the distal end 26 andthe shaft 28 may be selected based on the required strength of the nailand the intended use of the intramedullary nail. The nail 20 depicted inFIGS. 1-11 and 13-24 is generally sized and shaped for treating afracture toward the middle of an otherwise healthy adult human femur. Ifdesired, the nail 20 may be included in a kit having various sizes ofnails to fit the femurs of variously sized patients, and/or havingvarious sizes of nails to fit various types of femoral bone conditionsor various types of femoral fractures, and/or further having varioussizes of nails to fit various other bones. For instance, the length ofthe femoral nail 20 shown may be selected as needed between about 10 and20 inches.

The distal end 26 may include a tip 30 having for instance a conical orpartially conical profile. The conical profile of the tip 30 aids ininserting the nail 20 into the medullary canal. The shaft 28 may begenerally of constant diameter. The proximal end 24 may include aportion of larger diameter than the shaft 28.

As shown in FIGS. 4-6 and known in the art, the nail 20 has an overallcross-sectional shape selected based upon the intended use. For afemoral nail 20, the cross-sectional shape may be generally circular, tomatch the generally circular cross-sectional shape of the medullarycanal of a healthy femur. For instance, the shaft 28 may be generallyformed with an outside diameter of 0.394 inches.

As best shown in FIGS. 1, 4 and 6, shallow longitudinal recesses 32 maybe formed into the outside surface of the shaft 28. These longitudinalrecesses 32 help to increase blood supply through the endosteum of thebone and to the fracture site during healing. Other cross-sectionalshapes can be alternatively used for particular purposes or to bettermatch the cross-sectional shape of the medullary canal of the particularbone being treated.

A cannula 34 preferably extends the length of the nail 20. The cannula34 facilitates insertion and alignment of the nail 20 within themedullary canal. The cannula 34 may be formed at each of the ends 24, 26by drilling along the longitudinal axis 36 of the nail 20. In the shaft28 of the nail 20, the cannula 34 may be formed by cutting into the nail20 from one of the sides. Alternatively, the cannula 34 may be formed bydrilling longitudinally the entire length of the nail 20, which wouldresult in a shaft 28 which encloses the cannula 34. Because the naillength is great compared with the nail width, it is generally easier tofabricate the cannula 34 by cutting laterally through the side of theshaft 28 than by drilling the length of the nail 20.

The cannula 34 receives a guide wire (not shown) during insertion of thenail 20 into the medullary canal. The guide wire has to be thick enoughto provide the requisite strength and rigidity for placement into thebone, and the cannula 34 must be large enough to receive the guide wireand permit longitudinal travel of the nail 20 along the guide wire.Conversely, because a larger cannula 34 detracts from the strength ofthe nail 20, the cannula 34 should be as small as required for travelover the guide wire. The preferred guide wire is circular incross-section, and as shown in FIGS. 4-6 the preferred cannula 34generally matches this circular cross-section. For instance, the cannula34 may be about 0.156 inches in diameter. With a shaft 28 of 0.394 inch(10 mm) diameter, this cannula 34 leaves a wall thickness for the shaft28 of about 0.118 inches.

The preferred nail 20 includes a large radius bend 38 in the shaft 28,generally intended to match the anterior-posterior bend of a healthyfemur. The bend 38 may have a large radius in relation to the length ofthe nail 20, such as a bend with a radius of 2 to 10 times the length ofthe nail 20. The curvature of the bend 38 may be applied over only acentral portion of the shaft 28, leaving the proximal end 24 and distalend 26 straight. For instance, the bend 38 may be applied over a central5 to 13 inches of the nail 20, depending on nail length.

Other than the cannula 34 being open from only one side of the shaft 28,the nail 20 is preferably symmetrical about a bisectinganterior-posterior plane. This allows the nail 20 to be used in eitherthe right or left femur while still maintaining the bend 38 appropriatefor the curvature of the femur.

The nail structure 22 is formed of a structurally strong bio-compatiblematerial as known in the art. For instance, the nail structure 22 can beformed of a single piece of metal, with the preferred metal beingtitanium, such as a Ti-6AL-4V ELI titanium per ASTM F-136. For certainapplications, the bone support structure may be formed of abio-compatible ceramic or composite material of adequate strength andrigidity.

The proximal end 24 is preferably formed with one or more through-holes40 to facilitate attachment to a proximal bone fragment. For instance,the proximal end 24 may include two holes 40 which intersect each other.As best shown in FIG. 2, each of these holes 40 preferably extends at anangle a relative to the longitudinal axis 36 of the nail structure 22,with the preferred angle a being about 46°. Both the holes 40 preferablyextend at an anteversion angle β of about 15°, posteriorly (downward asshown in FIG. 1) on the proximal side and anteriorly (upward as shown inFIG. 1) on the distal side. These holes 40 allow attachment to a femoralfragment by bicortical attachment and with either antegrade fixation(i.e., through the trochanter) or reconstruction fixation (i.e., intothe femoral head) as selected by the orthopedic surgeon. Alternativelyor in conjunction with the through-holes 40, one or more recesses orcavities (not shown) may be provided in the proximal end 24 to permitunicortical attachment of the proximal end 24.

The proximal end 24 of the nail structure 22 may further includestructure to facilitate attachment of a drilling or aligning jig (notshown) as known in the art for placement of bone fasteners relative tothe nail 20. For instance, a proximal opening 42 aligned along thelongitudinal axis 36 may be used to receive an end of a jig in a matingrelationship. Workers skilled in the art will appreciate that numerousother structures could be equivalently used to temporarily hold the jigrelative to the nail 20.

The distal end 26 of the nail structure 22 includes at least onedynamization window 44 through an external surface, with a spacer 46 inthe dynamization window 44. The term “window” as used herein refers toan opening on an exterior surface of the nail 20. These windows 44 arereferred to as “dynamization” windows because, when used in conjunctionwith a properly dimensioned bone fastener (shown in FIGS. 18-24) andwith a spacer 46 formed of a bioresorbable material, the proportion ofstress carried by the nail 20 relative to stress carried by the healingbone across the fracture site dynamically changes as a function of time.

If desired, a single dynamization window or cavity may be provided,which would permit only unicortical attachment of the nail 20. In thepreferred embodiment, two dynamization windows 44 are provided onopposite sides of the nail 20, with each dynamization window 44extending entirely through the side wall of the nail 20 to permitcommunication with the cannula 34. After removal of the guide wire fromthe cannula 34, the dynamization windows 44 permit bi-corticalattachment by inserting a bone fastener through the cortex on one sideof the nail 20, “in” one dynamization window 44, “out” the otherdynamization window 44, and through the cortex on the other side of thenail 20. While the preferred embodiment includes only one set ofdynamization windows 44, additional dynamization windows may be locatedat other longitudinal locations of the nail 20, including the proximalend 24 and the shaft 28 as well as the distal end 26. Any additionaldynamization windows may either be single dynamization windowspermitting unicortical attachment or opposing sets permitting bicorticalattachment. Any additional dynamization windows may either beperpendicular to the bisecting plane or at other angles through the nail20.

In the preferred embodiment, the dynamization windows 44 are aligned onopposite sides of the nail 20 at the same longitudinal location. Withthis configuration, both dynamization windows 44 may be simultaneouslyformed by a single cutting tool advanced through the nail 20 in adirection perpendicular to the bisecting plane. Alternatively, twodynamization windows may be longitudinally (and/or radially) offset withrespect to each other and still permit bicortical attachment, providedthey sufficiently overlap to permit the bone fastener to simultaneouslypass through both windows.

A spacer 46 is placed in each dynamization window 44. During use of thenail 20 as shown in FIGS. 18-24, a bone fastener 48 is positioned intothe dynamization window 44, and the spacer 46 spaces the bone fastener48 relative to the nail structure 22 defining the dynamization window44. Force is transmitted between the nail structure 22 and the bonefastener 48 primarily as a compressive load on a portion of the spacer46.

Each spacer 46 is formed of a non-metal material, and preferably of abioresorbable material. The term “bioresorbable” as used herein refersto any biocompatible material which dissolves or degrades over timeafter implantation into the human body. Among others, possiblebioresorbable materials include polymers and copolymers glycolic acid,lactic acid, aminocaproic acid, lactides, desoxazon, hydroxybutric acid,hydroxyvaleric acid, hydroxymethacrylate, peptides, polyesters ofsuccinic acid and cross-linked hyaluronic acid, or even a biologicallyabsorbable hydroxyapatite or tricalcium phosphate. The preferredbioresorbable material is a polylactic acid (“PLA”), which provides astrong material for the spacers 46. The compressibility of the PLAmaterial shows little change over the first few weeks of implantation,but then increases linearly over the next few months until resorption tothe point where the material will no longer support a load. With thepreferred PLA material, full resorption will typically occur withinabout two to five years. If no bioresorption is desired, the non-metalmaterial may be any other polymer commonly used in medical implants,such as a preferred non-metal non-resorbable material of ultra-highmolecular weight polyethylene (“UHMWPE”).

The dynamization windows 44 and the spacers 46 are shaped based on therequired strength and the desired dynamization characteristics for thenail 20. In the preferred embodiment as shown in FIGS. 1 and 3, both thefirst and second dynamization windows 44 and spacers 46 have circularends 50 and a rectangular central section 52, and the spacers 46 fillthe dynamization windows 44 in length and width. As will be furtherexplained with reference to FIGS. 18-24, this shape provides substantiallongitudinal dynamization flexibility while still providing adequatestrength for the nail 20 at the dynamization windows 44. In the 0.394inch (10 mm) OD nail 20 and for use with 0.177 inch (4.5 mm) OD bonescrews 48, each end 50 may have a circular radius of 0.124 inches, withthe central section 52 being 0.345 inches in length and 0.248 inches inwidth, for a total dynamization window length of 0.611 inches. Similarsizes may be used in bone plates. Alternatively each spacer may notcompletely fill its dynamization window, such as by not being eitherfull width or full length (which controls whether force transmittedthrough the spacer is in compression, in tension or in shear), or byhaving a central opening through each spacer.

As best shown in cross-sectional views of FIGS. 2 and 5, each spacer 46has an exposed surface which may have a shallow groove 54 in the center.The groove 54 may provide a “V” shape to the exposed surface of thecentral section 52 of the spacer 46, while the exposed surface of theends 50 of the spacer 46 may be conical. In the preferred embodimentshown, the groove 54 is about 0.04 inches (1 mm) deeper than the edgesof the spacer 46. During surgical implantation of the bone fastener 48into the nail 20, the groove 54 assists in directing the transverseguide pin/drill/bone fastener inward toward the center of the spacer 46.Workers skilled in the art will appreciate that numerous alternativesurface contours may be selected for one or both spacers 46 which stillprovide a generally sloped surface directing the guide pin/drill/bonefastener inward toward a center of each spacer 46.

In the preferred embodiment as best shown in FIGS. 5 and 7, the twoopposing spacers 46 are formed as a single insert 56. For instance, witha 0.394 inch (10 mm) OD nail 20, the insert 56 may have an overallthickness of about 0.286 inches (7.3 mm). Alternatively, each spacer 46may be separately formed.

A cannula 58 is formed in the insert 56 to correspond with the cannula34 of the nail structure 22, such that the two spacers 46 are defined onopposing sides of the cannula 58. With the center of the groove 54 onthe outside and the cannula 58 toward the inside, the center of eachspacer 46 may be quite thin. For instance, with a cannula 58 of about0.156 inches (3.9 mm) in diameter, the center of each spacer 46 may beonly about 0.025 inches (0.6 mm) thick.

As an alternative to the groove 54, the spacer 46 may include an exposedsurface which is planar. Depending upon the material of the spacer 46and the thickness of the spacer 46 relative to the cannula 58, thecenter of the spacer 46 may be resiliently deflected or deformed inwardunder pressure. For instance, the push force placed on the spacer 46 bythe guide pin, drill and/or bone fastener during insertion through thespacer 46 may cause the center of the spacer 46 to resiliently deform,such that an exposed surface which was planar as manufactured provides asloping profile which assists in directing the guide pin/drill/bonefastener toward the center of the spacer 46.

The preferred bioresorbable material is commercially available such asin about 150 in³ blocks. The insert 56 may be formed by cutting thebioresorbable material to ⅝ inch by ⅝ inch by 3 inch portions, which maythen be further fabricated to the shape of the insert 56 by CNC. Thecannula 58 is preferably drilled into the insert 56 prior to insertionof the insert 56 into the nail 20, although the cannula 58 mayalternatively be drilled after placing the insert 56 into the nail 20,either simultaneously with or after formation of the cannula 34 throughthe nail structure 22.

The insert 56 may fit into the dynamization windows 44 with a press fit.The insert 56 is pressed in the nail 20 until it aligns centrally withinthe nail 20. Initial results have indicated that several hundreds ofpounds of press force is required to press the preferred insert 56 intothe windows 44 of the preferred nail structure 22. During the surgery,the insert 56 can be drilled through with a push force which is at leastan order of magnitude less than the press force, and the press fit amplysecures the insert 56 into the nail 20. For example, the press fit mayhave a pull out force of 50 pounds or more.

One way to form the press fit is to oversize the spacer 46, such as fromone to several mils, in all dimensions relative to the windows 44. Thepress fit then creates a static compressive stress which is relativelyuniform in all directions within the spacer 46. For at least somebioresorbable materials, it is believed that the amount of compressivestress changes the resorption rate and/or breakdown of the material. Theamount of compressive stress and the direction of the compressive stresscan thus affect the controllability and uniformity of increasingdynamization as a function of time.

During use after implantation, the intramedullary nail 20 is regularlyloaded in compression and then unloaded, i.e., “longitudinal compressivecycling”, such as when the healing bone supports the weight of thepatient during walking. Tensile stresses and bending stresses, whileoccurring in the bone depending upon what the patient is doing, occurmuch less often and much less regularly than compressive stresses. Forat least some bioresorbable materials, the typical longitudinalcompressive cycling of an insert will also affect the dynamizationprofile. With an understanding of typical longitudinal compressivecycling of the insert, the amount of static compression and thedirection of static compression created by the press fit can be selectedto enhance the dynamization profile. In particular, the insert 56 canhave a length which mates with the windows 44 with a firstinterference/clearance, and a width which mates with the windows 44 witha second, different interference/clearance. The preferred press fitplaces a static compression in the width direction of the insert 56, butno static compression in the length direction of the insert 56. That is,the insert 56 is oversized in the width (transverse) direction comparedto the windows 44, but the length of the insert 56 matches the length ofthe windows 44 or leaves a slight clearance so there is no staticcompression stress in the longitudinal direction due to the press fit.The preferred width oversize is about 1 to 10 mils, or more preferablyabout 2 to 4 mils. The preferred width oversize provides a maximumstatic compressive stress in the transverse direction on the order of 30to 80% of the yield stress of the preferred material of about 115 MPa,with no static compressive stress in the longitudinal direction.

There are other features which can be enhanced by the way the spacers 46are attached into the dynamization windows 44. Various recesses orprotrusions on the spacers 46 and/or in the nail structure 22 mayprovide a higher pull strength or facilitate a positively securedattachment of the spacers 46 to the nail structure 22. One example ofthis is depicted in the alternative insert 156 of FIGS. 8 and 9. In thisembodiment, the insert 156 has ridges 176 extending around a portion ofthe insert periphery 178 which makes contact with the nail structure 22.The ridges 176 form an interference profile relative to the windows 44.The preferred ridges 176 are about 5 mils thick, extending only aroundthe semi-cylindrical ends of the insert 156 to add about 10 mils to thelongitudinal length of the insert 156.

When the insert 156 is inserted into the nail structure 22, the ridges176 make interference contact with the windows 44 in the nail structure22. The non-metal material of the insert 156 has a highercompressibility than the metal of the nail structure 22, and due to thisinterference the ridges 176 compress inward upon insertion into the nailstructure 22 and place internal compression stresses on the insert 156.Because the ridges 176 occupy some but not all of the external periphery178 of the insert 156 which contacts the nail structure, the compressivestresses caused by the ridges 176 differ locally over the exterior faceof the insert 156. The width of the ridges 176 may be selected basedupon the compression desired, such as a width within the range of about5 to 50 mils. The compression of the ridges 176 thus forms one mechanismto more securely hold the spacers 46 of the insert 156 in place.

By having ridges 176 only at the longitudinal (i.e., proximal anddistal) ends of the insert 156, the static compression of the ridges 176due to the press fit occurs primarily in the longitudinal direction. Theamount of static compression in both the transverse direction and thelongitudinal direction can still be controlled. For example, the insert156 can be generally oversized in the width direction, while only theridges 176 are oversized in the longitudinal direction.

With a resorption rate that differs as a function of local compressivestress, the ridges 176 may bioresorb at a different rate than the restof the insert structure. The location of the ridges 176 can be selectedas desired, either closer to contact to bodily fluids or more removedwithin the nail structure 22, to further affect how the ridges 176resorb. If desired, the ridges 176 can located and sized such thatresorption of the ridges 176 is the primary mechanism for increasingdynamization of the fracture site. If desired, the nail structure 22 canbe modified to include ridges (not shown) rather than including theridges 176 on the insert 156, producing the same general effect ofcompressive stresses which differ locally over the exterior face of theinsert.

As yet another option, the nail structure 22 can be modified to includerecesses (not shown in FIGS. 8 and 9) which correspond in location tothe ridges 176 on the insert 156 of FIGS. 8 and 9. If correspondingrecesses are formed into an alternative nail structure, the insert 156can be received into the alternative nail structure with a “snap fit”.That is, during transverse pressing of the insert 156 into thealternative nail structure, the ridges 176 will be compressed inwarduntil the ridges 176 snap outward into the corresponding recesses in thealternative nail structure. With such a snap fit, the ridges 176 do notcontribute to the local compression stress profile of the insert 156,but rather positively lock the insert 156 into the alternative nailstructure to prevent push out during transverse drilling and/oradvancing the transverse bone fastener through the insert 156. Similarlyto ridges 176, the nail structure 22 can be modified to include front,back, or front and back ridges or lips (not shown) which would preventpush-through and/or pull-out of the insert 56, 156 from the windows 44.

A “snap fit” example is depicted in the alternative insert 556 of FIGS.10 and 11. In this embodiment, the insert 556 has bump ridges 576extending around a portion of the insert periphery 578 which makescontact with the nail structure 522. As best shown in FIG. 11, thewindow 544 includes a mouth portion 580, a neck portion 582, and areceiving notch 584. The bump ridges 576 form an interference profilerelative to the neck portion 582. For instance, other than the bumpridges 576, the preferred insert 556 has a width which is nominally 2mils narrower than the neck portion 582, permitting a slight clearanceduring insertion of the insert 556 into the window 544. The width of thebump ridges 576 may be selected based upon the desired push force toinsert the insert 556 into the nail 522, and based upon the desiredamount to interference to prevent the insert 556 from being undesirablyremoved from the nail 522, such as a bump ridge width within the rangeof about 5 to 50 mils. The preferred bump ridges 576 are about 20 milswider than the rest of the insert 556, providing an interferencerelative to the neck portion 582 of about 9 mils per side. The preferredbump ridges 576 are circular in profile, concentric with the cannula534. The preferred mouth portion 580 widens slightly relative to theneck portion 582, such as at an angle 586 of about 30°.

Insertion of the insert 556 into the nail structure 522 is simple andstraight-forward. First, the insert 556 is positioned and orientedimmediately adjacent the window 544. Positioning of the insert 556 canbe performed by hand or with a pliers, tweezers or similar graspingtool. The widening of the mouth portion 580 assists in aligning theinsert 556 relative to the window 544. Once aligned with the axis of thewindow 544, the insert 556 can be freely advanced into the nailstructure 522 until the bump ridges 576 contact the mouth portion 580,i.e., to the position shown in FIG. 11. In this position, the bumpridges 576 begin to make interference contact with the mouth portion580. The bump ridges 576 thus represent a trailing portion of thesurface of the insert 556 which makes contact with the nail structure522 upon insertion. In contrast to the rest of the contact surface ofthe insert 556 which extends parallel to the insertion axis, the bumpridges 576 extend at angles relative to the insertion axis.

The preferred bump ridges 576 are provided with a radius and curvaturewhich roughly corresponds to the angle of the mouth portion 580. Thatis, the preferred bump ridges 576 have about a 60° arc portion whichextends beyond the rectangular profile (in cross-section) of the rest ofthe contact surface of the insert 556, with about a 150° intersection orcorner at the leading side of each bump ridge 576 and about a 150°intersection or corner at the trailing side of each bump ridge 576. Thenon-metal material of the insert 556 has a higher compressibility thanthe nail structure 522, and, when the insert 556 is forced into the nailstructure 522 as shown by arrows 588, the angle 586 of the mouth portion580 compresses the bump ridges 576 inward. Because the bump ridges 576are concentric with the cannula 532, the sidewall 590 adjacent the bumpridge 576 can further accept the interference by deflecting inward intothe cannula 532. In the preferred embodiment, this sidewall 590 has awall thickness of about 0.04 inches.

The insert 556 is pushed further into the nail structure 522, until thebump ridges 576 snap outward into the receiving notches 584. In thepreferred embodiment, the receiving notches 584 have a cornered profile.The cornered profile does not compress the bump ridges 576 nearly asefficiently as the angle 586 of the mouth portion 580. Thus, once theinsert 556 is snapped into place within the nail structure 522, the pushforce required to remove the insert 556 from the nail structure 522 maybe several times the push force which was initially required to place inthe insert 556 into the nail structure 522. In the preferred embodiment,the receiving notches 584 still provide a slight interference with theinsert 556, such as 2-3 mils interference. This slight interferencerestricts movement of the insert 556 relative to the nail structure 522,despite the slight clearance between the insert 556 and the neckportions 582.

The insert 556 and the nail structure 522 shown are symmetrical about aplane perpendicular to the push or insertion axis. Thus, it is equallypossible to press the insert in from either side of the nail structure.Similarly, the insert may be pushed into place with either side pushedin first (i.e., orienting the insert 556 right-side-up as shown in FIG.11 or upside-down does not matter). If desired, the bump ridges 576, thereceiving notches 584 or both may be formed non-symmetrically. Forinstance, if desired the bump ridges 576 may be formed with a squaredback end, of a shape and size to mate like a barb with the corner of thereceiving notches 584. By forming a barb-type interference between thebump ridges 576 and the receiving notches 584, an even higher “pull out”force can be obtained with a lower “push in” force. Workers skilled inthe art will thus appreciate that many other shapes can be used toprovide a snap fit to the insert.

In addition to the nail 22, 522 shown in FIGS. 1-11 and 13-24, thepresent invention also has a wide range of applicability in other bonesupport implant devices, such as in a bone plate 94 as shownparticularly in FIG. 12. Even though the present invention is describedlargely with reference to the intramedullary nail structure 22, 522 muchof the description applicable to the intramedulary nail structure 22,522 of FIGS. 1-11 and 13-24 is equally applicable to bone plates orother bone support implant devices.

The bone plate 94 shown in FIG. 12 has a top surface 95 and abone-contact surface 96. The bone plate 94 has an overallcross-sectional shape selected as known in the art based upon theintended use of the bone plate 94. A plurality of through-holes 97extend through the bone plate 94. The holes 97 are provided to receivebone fasteners to secure the bone plate 94 to portions of the bone (notshown). If desired, the holes 96 may have a surface profile as known inthe art which interacts with the head of a bone fastener to apply eithercompressive or tensile stress across a fracture site.

In a preferred embodiment, each of the holes 97 are similarly shaped. Aplurality of inserts 556 are included in a bone support kit. Based uponthe fracture site and/or the desired treatment modality, the surgeonthen selects which holes 97 should be filled with inserts 556 and whichinserts 556 should be used. In particular, one or more holes 97 may beleft open, while one or more other holes 97 are filled with inserts 556.The use of bone fasteners in insert-filled holes allows dynamizationacross the fracture site as desired by the surgeon. The use of bonefasteners properly positioned in open holes 96 may prevent dynamizationuntil those bone fasteners are subsequently removed.

Bone plates are utilized on the surface of the bone, with bone fastenerswhich extend through the bone plate into the bone. In contrast to nails,i.e., because bone fasteners when used with bone plates do not extendthrough the bone prior to reaching the bone support implant, issuespreviously discussed with regard to unicortical or bicortical attachmentare less significant with regard to bone plates. The bone plate 94 isattached with bone fasteners inserted through the holes 97 in the boneplate 94 and secured into bone. The holes 97 in the bone plate 94 willaccordingly always be through-holes, not cavities.

Because bone plates are utilized on the surface of the bone, use of aguide wire within a longitudinally extending cannula is generallyunnecessary for bone plates. However, the preferred insert used with abone plate 94 may still retain a longitudinally extending cannula 532,even if no corresponding cannula is machined into the plate structure94. As when used with a cannulated nail 20, the central cannula 532 inthe insert 556 places the bump ridges 576 on a relatively thin sidewall590, which can be bent/compressed inward during insertion of the insert556 into the bone plate 94.

Attachment of the insert(s) into the dynamization window(s) does nothave to be performed as a manufacturing step. Alternatively, the surgeonmay attach the inserts into the dynamization windows as a preparatorystep during surgery, and the bone support implant and inserts may beappropriately modified to facilitate placement of the insert(s) into thedynamization window(s) by the surgeon. As used herein, the term“surgeon” includes any treating physician or any treatment professionalunder the direction of a surgeon or other treating physician. Forinstance, the insert(s) and dynamization windows may have a smalleramount of interference to enable the surgeon the press the insert(s)into the bone support implant by hand. Alternatively, the surgeon may beprovided with a mechanical press to facilitate pressing the insert(s)into the bone support implant. If the insert has an interference profileso as to be received in the bone support implant with a snap fit, thesurgeon obtains the additional comfort of knowing the insert is properlypositioned when the insert snaps into place.

If desired, a lubricant may be utilized to facilitate the press fit orsnap fit. The lubricant used may be volatile, so the insert becomestightly secured into the bone support implant after the lubricantevaporates. As another alternative, the insert and the dynamizationwindows may be sized with a slight clearance and be adhesively secured.Any lubricant or adhesive should be biocompatible so as to not createcomplications in the healing process.

Attachment of the insert into the dynamization windows by the surgeonallows several further advantages. For instance, a single bone supportimplant may be provided as part of a kit which includes a plurality ofinserts having different properties. The different inserts provided mayhave different mechanical properties, such as different hardnesses,different rates of absorption, etc., allowing the surgeon theflexibility to match the insert used with the particular healingmodality desired by the surgeon. One or more of the inserts in the kitmay be bioresorbable, while one or more other inserts in the kit arenot. Thus, the surgeon may select whether dynamization occurs at all.One preferred kit includes a first insert which starts dynamization attwo to four weeks after implantation and fully dynamizes after ten totwelve weeks, a second insert which starts dynamization at eight to tenweeks and fully dynamizes after about sixteen weeks, and a third insertwhich does not bioresorb. Each of the differing inserts in the kit ispreferably marked or color-coded so the surgeon can quickly identifywhich insert has the desired mechanical or chemical treatmentproperties.

The inserts may also include one or more active agents to promoteeffective healing. For instance, the non-metal material of the insertsmay include one or more antibiotics such as gentamicin, methicillin,penicillin, etc. The non-metal material of the insert may also includeother active agents, such a one or more of a transforming growthfactor-beta 1, a recombinant human bone morphogenetic protein-2, etc. Ifprovided as part of a kit, different inserts may be provided each with adifferent active agent or a different amount of active agent, so thesurgeon can select the type and amount of active agent used for theparticular surgery.

Additional flexibility is provided if the bone support implant hasmultiple sets of dynamization windows 44. If the bone support implanthas multiple sets of dynamization windows 44, the surgeon may elect topress inserts into less than all of the windows 44, or to press insertshaving different physical or mechanical properties into the variousdynamization windows 44.

Another advantage of attachment of the insert into the dynamizationwindows 44 by the surgeon is that the insert may be handled in adifferent environment from the nail structure 22. For instance, theinsert may be maintained in a particular thermal condition (e.g.,refrigerated or frozen), or in a sealed container (e.g., sealed fromair, sealed from humidity, etc.) until immediately prior to insertioninto the dynamization windows 44 and immediately prior to implantationinto the fractured bone. The controlled environment of the insert mayhave beneficial results in physical properties (e.g., preventingdissipation or dilution of an active agent, etc.) or in mechanicalproperties (e.g., increased hardness, different size due to thermalexpansion, etc.) of the insert upon implantation.

FIG. 13 depicts one preferred container 180 during assembly to be sealedabout the insert 56. The preferred container 180 is a double layerpouch. An inner pouch 182 is sealed around the insert 56 and formed suchas of PET, aluminum foil and polyethylene or polypropylene. An outerpouch 184 is sealed around the inner pouch 182 and formed such as ofTYVEK spun bond polyethylene, paper, polyester and/or polyethylene. Thesealed container 180 is specially designed to maintain sterility of theinsert 56 until use and to increase the shelf-life of the insert 56. Inparticular, the sealed container 180 substantially prevents the insert56 from contacting germs, air, and moisture or humidity. The foil and/orpaper shields the insert 56 from light. The foil and/or paper can alsoinclude printing such as identifying the insert 56 and instructionalinformation. Depending upon the material selected for the insert 56,water absorption from humidity, oxidation, or light degradation of thepolymer may affect the dynamization profile for the insert 56. In thatthe Insert 56 should have a consistent dynamization profile regardlessof the length of time the insert 56 has sat on the shelf prior toplacement into the nail structure 22 and implantation, the sealedcontainer 180 is important for shelf-life. For instance, the container180 may be flushed with nitrogen upon sealing, such that the insert 56is retained in a nitrogen environment for prolonged shelf-life. Thedouble-layer pouch 180 facilitates use of the insert 56 in a sterileoperating theater.

Prior to sealing the insert 56 in the container 180, the insert 56should be sterilized. One method of sterilization is through Cobalt 60Gamma irradiation, such as at about 2.5 to 4 Mrad or a dose of about 25to 40 kGy. Gamma irradiation sterilization changes the morphology of thepreferred bioresorbable material, such as through chain-scission orcross-linking, which causes some reduction of average molecular weight.Of particular importance, the gamma irradiation increases the rate ofdegradation of the preferred bioresorbable material, and thus theeffects of the gamma irradiation must be taken into account in selectingthe insert material for a desired dynamization profile. A second methodof sterilization is through ethylene oxide gas sterilization, which isbelieved not to significantly affect the dynamization profile. A thirdmethod of sterilization is through gas plasma sterilization, which isbelieved to result in a slower dynamization profile on the preferredmaterial than gamma irradiation. Gas plasma sterilization is alsoappropriate for inserts of non-resorbable materials such as polyethylene(low molecular weight or UHMWPE). After sterilization and prior tosealing in the container 180, the insert may be dried such as throughvacuum drying.

FIG. 14 shows a third alternative insert 256 positioned for insertion ina corresponding nail structure 222. As shown, this insert 256 and itswindow 244 have differing shapes between distal end 250 and proximal end251. The proximal end 251 of the insert 256 transmits compressive loadsto the nail structure 222, whereas the distal end 250 of the insert 256transmits tensile loads to the nail structure 222. The differencebetween shapes at the proximal and distal ends 251, 250 is particularlyappropriate for weight bearing bones such as the femur because suchweight bearing bones are much more often loaded in compression than intension. The squared off proximal end 251 of the insert 256 transmitscompressive stress across a wider surface area than the semi-cylindricalproximal end 50 of FIGS. 1-13. The squared off proximal end 251 of theinsert 256 also has a more uniform compressive stress load across itswidth, rather than concentrating the compressive stress load along thecenterline of the insert 56. The third alternative insert 256 is alsoapplicable for use with bone plates or other bone support implants.

Further along the lines that the spacer or insert, for certain bones,will rarely transmit tensile stresses, FIGS. 15 and 16 show an axialinsert 356. FIG. 15 shows the axial insert 356 aligned for axialinsertion into a corresponding insert reception recess 386 in a distalend 26 of a nail structure 322. During assembly, the axial insert 356 isadvanced axially into the insert reception recess 386. Assembly may beperformed either as a manufacturing step or by the surgeon immediatelyprior to implantation. Depending upon which bones are being treated andthe desired treatment modality, the axial insert insert 356 may also beused with bone plates or other bone support implants.

With axial insertion, the axial insert 356 can be sized significantlylarger and/or longer than the window 44, so there is substantially nopossibility of a transverse push-out of the axial insert 356 such as dueto the drill force. For example, the axial insert 356 has proximal anddistal extensions 388, 390 around a spacer portion 346. When the axialinsert 356 is positioned in the insert reception recess 386, only thespacer portion 346 is visible in the windows 44. When the axial insert356 is positioned in the insert reception recess 386, the proximal anddistal extensions 388, 390 project beyond the proximal and distal endsof the windows 44. For instance, each of the proximal and distalextensions 388, 390 may be 0.05 inches or longer in length. In thepreferred axial insert 356 shown in FIG. 15, the axial insert 356 isabout 0.9 inches in length, while the windows 44 are only about 0.55inches in length. When fully inserted, the proximal extension 388extends proximally about 0.05 inches past the proximal end of thewindows 44, and the distal extension 390 extends distally to the end ofthe nail structure 322, about 0.25 inches beyond the distal end of thewindows 44.

The proximal end 388 of the axial insert 356 abuts against the proximalend 392 of the receiving opening 386 in the nail structure 322 totransmit compressive loads to the nail structure 322. Thus, in a nailstructure 322 with a cannula 34, the axial insert 356 should havetransverse dimensions which are significantly greater than the diameterof the cannula 34, to transmit the compressive stress across sufficientsurface area. The axial insert 356 and the corresponding receivingopening 386 in the distal end 26 of the nail structure 322 may be formedin simple cylindrical shapes. For instance, in a nail structure 322 witha 0.12 inch diameter cannula 34, the axial insert 356 may have about a0.25 inch diameter. This leaves a surface area of (0.25²-0.12²)π/4square inches to support the compressive load placed upon the fracturedbone. A 0.12 inch diameter cannula 58 also extends through the axialinsert 356, so the guide wire (not shown) can be used in the traditionalmanner.

As mentioned previously, intramedullary nails transmit tensile forcesmuch less frequently than compressive forces, and the tensile forcestransmitted are typically much smaller in magnitude. Still, the axialinsert 356 should have some interference which will prevent the axialinsert 356 from freely moving out of the corresponding receiving opening386 in the distal direction if a tensile force is being transmitted. Forexample, the axial insert 356 may be formed with a 2 to 10 milinterference in the receiving opening 386, and retained with a pressfit. The press fit will only place a static compressive load on theaxial insert 356 in the transverse direction, and thus will have onlyminor impact on the degradation rate of a bioresorbable material for theaxial insert 356.

If desired, the axial insert 356 may be formed with a spacer portion 346which is slightly larger in diameter than the distal extension 390. Forinstance, the distal extension 390 may be formed with a diameter whichmatches the diameter of the receiving opening 386, and the spacerportions 346 of the axial insert 356 may be formed having a diameterwhich is 2 to 10 mils larger and in the shape of the windows 44. Withsuch a configuration, axial insertion of the axial insert 356 into thereceiving opening 386 is only achieved with a press force whichcompresses the spacer portion 346 of the axial insert 356 radiallyinward. Then, when fully inserted into the nail structure 322, thespacer portion 346 uncompresses and springs radially outward into thewindows 44, locking the axial insert 356 from sliding distally in thenail structure 322.

FIG. 17 shows an axial insert 456 which has a non-circular shape intransverse cross-section. With a square cross-sectional shape, the axialinsert 456 better supports twisting stress on the intramedullary nail422. That is, in contrast to the cylindrical insert 356 of FIGS. 15 and16, the square cross-sectional shape prevents the axial insert 456 fromrotating about the longitudinal axis 36. Other shapes which arenon-circular in transverse cross-section can be similarly used.

As shown in FIGS. 1-3, the distal end 26 of the preferred nail 20preferably includes a non-dynamic through-hole 60. The through-hole 60has an axis 62 which is preferably perpendicular to theanterior-posterior plane and intersecting the longitudinal axis 36 ofthe nail 20. The through-hole 60 defines a first window 64 into thecannula 34 and a second window 64 out of the cannula 34 at the oppositeside of the nail 20. Each window 64 may be circular in cross-section,and both windows 64 may be defined with a single drilling operation. Thesize and shape of the windows 64 are selected based on the intended bonefasteners to be used. For instance, both windows 64 may be circular witha 0.217 inch diameter. For bi-cortical attachment of the distal end 26of the nail structure 22 using the through-hole 60, a bone fastener 48is advanced through the through-hole 60, i.e., through both windows 64.While the preferred embodiment includes only one set of non-dynamizationwindows 64, additional non-dynamization windows 64 may be located atother longitudinal locations of the nail 20, including the proximal end24 and the shaft 28 as well as the distal end 26. Any additionalnon-dynamization windows may either be single windows permittingunicortical attachment or opposing sets permitting bicorticalattachment. Any additional non-dynamization windows may either beperpendicular to the bisecting plane or at other angles through the nail20. Similarly, non-dynamization holes may be provided through the boneplate or other bone support implant.

The bone fasteners 48 used with the bone support implant 20 may be forinstance bone pins or bone screws, sized and shaped as appropriate forthe site of implantation. Each bone fastener 48 may be directlyimplanted into the cortex, or a hole may be drilled or otherwise openedin the cortex prior to placement of the bone fastener 48. The bone pinor bone screw may be solid, or may be cannulated such as forimplantation over a guide pin. In the preferred embodiment, the distalthrough-hole 60 is sized to receive 0.177 inch (4.5 mm) outside diameterbone screws, and the dynamization windows 44 and spacers 46 are sizedappropriately for 0.177 inch (4.5 mm) outside diameter bone screws. Theproximal through-holes 40 as preferably sized appropriately for 0.256inch (6.5 mm) outside diameter bone screws. Other types of bonefasteners may be alternatively used at the option of the orthopedicsurgeon.

FIGS. 18-24 show various attachment configurations for the nail 20 ofthe present invention. Though not shown in separate figures, the variousattachment configurations shown in FIGS. 18-24 are also generallyapplicable for bone plates. FIGS. 18 and 19 show a bicortical attachmentwith a single bone screw 48 positioned at the distal end of the twodynamization windows 44, which can be characterized as an “initialdynamic” locking position. Attached in this position, the nail 20provides only compressive dynamization across the fracture site 66, asfollows. The bioresorbable spacer 46 can be thought of as a compressionspring with a time-varying spring constant, positioned within asubstantially incompressible nail structure 22. In the attachment shownin FIGS. 18 and 19, substantially the entire length of the “spring” ison the proximal side of the bone fastener 48. Very little force istransmitted through the nail 20 until the bone is loaded. When thefractured bone is loaded in compression, the compressive load is carriedacross the fracture site 66 by the nail shaft 28 and then through theproximal length of the spacer 46, and then to the bone fastener 48 anddistal fragment 68. Initially on implantation, the bioresorbable spacer46 is very rigid and hard, and substantially incompressible like thenail structure 22. The nail 20 will carry substantially all of thecompressive force, and none of the compressive force will be carriedacross the fracture site 66.

After the bone begins healing, such as after several weeks, thebioresorbable material begins to deteriorate. This increases thecompressibility (lowers the spring constant) of the bioresorbablematerial in the dynamization window 44. In this condition, when acompressive stress is placed across the fracture site 66, the proximalside of the spacer 46 will compress slightly under the load. Because ofthis slight compression, significant amounts of the compressive stresswill be carried by the healing bone as well as by the nail 20.

As the bioresorable material further deteriorates, the proportion ofstress carried by the nail 20 relative to stress carried by the healingbone continues to decrease. The healing bone continues to be dynamized,until substantially all compressive stresses placed on the bone arecarried across the fracture site 66 rather than by the nail 20.

Most of the stresses carried by the bone are compressive stresses ratherthan tensile stresses. Nonetheless, in contrast to the compressivedynamization, consider the path of tensile stress placed on the bonewhen the nail 20 is attached as shown in FIGS. 18 and 19. When the boneis loaded in tension, the tensile stress is carried across the fracturesite 66 by the nail shaft 28 and then around to the distal side of thedynamization window 44 by the nail structure 22, then transferred as acompressive stress through only a small distal length of the spacer 46,and then to the bone fastener 48 and distal fragment 68. Because thebone fastener 48 is quite close to the distal end of the dynamizationwindow 44, there is a very short length of bioresorbable material toundergo compression, and there is very little give in the short distallength of bioresorbable material regardless of the amount ofdeterioration. Tensile stresses placed across the fracture site 66 arealmost entirely borne by the nail 20, regardless of deterioration of thebioresorbable spacer 46.

FIG. 20 shows an alternative attachment of the nail 20, which can beeither a “static” locking position or a “delayed dynamic” lockingposition depending upon screw removal. In this static locking position,the nail 20 is attached with a first bone screw 48 through the openthrough-hole 60 and a second bone screw 48 through a distal end of thedynamization windows 44. The two screw attachment helps further securethe distal fragment 68 to the nail 20, and particularly helps to preventany rotational movement or “toggling” of the distal fragment 68 whichmight otherwise occur about a single screw. Toggling of the distalfragment 68 may particularly be a problem if the distal end 26 of thenail 20 does not fit securely and tightly within the medullary canal ofthe distal fragment 68.

With two screw attachments and particularly with the screw 48 throughthe open through-hole 60, there is very little dynamization which isinitially seen by the fracture. However, an intermittent operation maybe performed after initial healing of the fracture in which the bonescrew 48 through the open though-hole 60 is removed, resulting in thedelayed dynamic configuration. With a single screw attachment throughthe distal end of the dynamization windows 44, compressive dynamizationof the fracture will be achieved after the intermittent operation.

If a completely static attachment is desired, the recommendedpositioning of bone screws 48 includes a first screw 48 through the openthrough-hole 60 and a second bone screw 48 through a proximal end of thedynamization windows 44 as shown in FIG. 21. This positioning allowsmaximum separation between the bone screws 48 for toggle prevention andmaximum strength. For each of the initial dynamic, the delayed dynamicand the completely static attachments, the surgeon can further adjustbone screw positioning as necessary for the condition of the bone.

In an alternative nail design (not shown) having two distal sets ofdynamization windows 44, toggling of the distal fragment 68 will beprevented by a two screw attachment while full dynamization can beachieved without removal of either screw.

Many middle grounds or intermediate longitudinal locations can also beselected by the surgeon for placement of the bone screw 48 through thedynamization windows 44. By selecting the longitudinal location of thebone screw 48 through the dynamization windows 44, the surgeon canselect the proportion of compressive dynamization and tensiledynamization seen at the fracture site 66.

The dynamization windows 44 are significantly longer than the width ofthe intended bone fastener 48. Because of this, while the exactlongitudinal location of the bone fastener 48 is important for thedesired dynamization, the exact longitudinal location is not critical touse of the nail 20. Minor longitudinal displacement errors of the bonefastener 48 will not prevent the bone fastener 48 from being advancedthrough the nail 20. The preferred nail structure 22 permitslongitudinal displacement of the preferred bone fastener 48 up to amaximum of 0.434 inches while still receiving the bone fastener 48through both windows 44. This large range of longitudinal location ofthe bone fastener 48 relative to the dynamization windows 44 not onlyprovides permissible error for the surgeon, but also allows the surgeonflexibility in placement of the bone fasteners 48 relative to thefracture and relative to changes in bone condition at differentlongitudinal locations.

FIGS. 22-24 further show how the present invention provides flexibilityin locating the bone fastener(s) 48 relative to the intramedullary nail20 and also in providing for a range of error in locating the bonefastener(s) 48 relative to the nail 20. These benefits are achieved dueto the different mechanical properties (such as hardness) of thenon-metal material of the spacers 46, regardless of whether thenon-metal material chosen is bioresorbable.

The longitudinal length of the two windows 44 with respect to each otherallows for a significant longitudinal angulation γ of the bone screw 48relative to the nail 20, such as up to about 45° as shown in FIG. 22.Three factors may result in the longitudinal angulation γ of the bonescrew 48. Firstly, the location of the bone fastener 48 shown in FIG. 22may result in a bending dynamization of the fracture site 66. The bonefastener 48 contacts the nail 20 at a proximal end 70 of onedynamization window 44 and at a distal end 72 of the other dynamizationwindow 44. Tensile loads are transmitted through the distal end 72contact without dynamization, and compressive loads are transmittedthrough the proximal end 70 contact without dynamization. However,bending stress such as that created by placing a clockwise (in FIG. 22)moment on the distal fragment 68 may allow dynamization. The extent ofbending dynamization of the fracture site 66 depends on how secure thedistal end 26 is in the medullary canal of the distal fragment 68. Aloose fit of the distal end 26 in the distal fragment 68 will allow somerotational play, and the compressibility of the spacer material willgovern how much bending stress is transferred through the fracture.Conversely, a tight fit of the distal end 26 in the distal fragment 68will prevent any clockwise bending dynamization, as the distal fragment68 cannot rotate relative to the nail 20 due to the tight fit. A loosefit of the distal end 26 in the distal fragment 68 may result eitherfrom the condition of the original bone or due to widening the medullarycanal during surgery relative to the diameter of the nail 20. If thesurgeon wishes clockwise bending dynamization to occur, first a loosefit must be obtained, and then the bone fastener 48 is placed throughthe dynamization windows 44 as shown in FIG. 22. Through properlongitudinal angulation γ of the bone fastener 48, the structure of thepreferred nail 20 thus allows the surgeon to select whether, how much,and in which direction bending dynamization occurs.

A second reason for longitudinal angulation γ of the bone fastener 48 isbased on the condition of the fracture. With longitudinal angulation γof the bone fastener 48, the bone fastener 48 extends through one sideof the cortex at a position longitudinally offset from the location thebone fastener 48 extends through the other side of the cortex. Thesurgeon may determine that significant longitudinal angulation γ isnecessary for best securement of the bone fastener 48 relative to thefracture location(s).

A third reason for longitudinal angulation γ of the bone fastener 48 ismerely due to longitudinal angular misalignment of the bone fastener 48.The axis of the bone fastener 48 may be angularly misaligned relative toits desired position. The structure of the preferred nail 20 permitslongitudinal angular misalignment of the bone fastener 48 while stillreceiving the bone fastener 48 through both windows 44.

As best shown in FIG. 23, the width of the two windows 44 is preferablygreater than the width of the bone fastener 48. This difference in widthpermits some transverse displacement 74 of the bone screw 48 withrespect to the longitudinal axis 36 of the nail 20, either by error oras intended by the surgeon. The structure of the preferred nail 20 inconjunction with the preferred bone fastener 48 permits a transversedisplacement 74 up to a maximum of 0.071 inches. Because the spacermaterial is drilled in-situ or the bone fastener 48 used opens its ownhole through the spacer 46, the spacer 46 holds the bone fastener 48securely with respect to the nail 20 anywhere within the dynamizationwindows 44, at least until resorption of the spacer 46 becomessignificant.

As best shown in FIG. 24, because the width of the windows 44 is greaterthan the width of the bone screw 48, some amount of transverseangulation δ is also permitted. Similar to transverse displacement 74,this transverse angulation δ may either be the result of error or beintended by the surgeon. The structure of the preferred nail 20 permitsa transverse angulation δ with the preferred bone fastener 48 up to amaximum of about 11° from the axis of the dynamization windows 44.

The preferred PLA material for the spacers 46 and the preferred shape ofthe spacers 46 provide very useful general purpose dynamizationcharacteristics based on currently known information about how bonefractures heal. The present invention further introduces an entirely newscience to bone healing. That is, as explained with regard to thepreferred embodiment, the selection of the bioresorbable materialdetermines its compressibility curve as a function of resorption time.Different bioresorbable materials have different compressibility curves,affecting the dynamization seen at the fracture site 66. Differentspacer geometries and different bone fastener locations and geometriesalso affect the dynamization (tensile, compressive and bending) seen atthe fracture site 66. The present invention will allow a new body ofdata to be gathered on the effectiveness of bone fracture healing underdifferent dynamization conditions. Based on this data, future changesmay be made to further improve the invention, or to modify the inventionfor particular bone or fracture conditions. For instance, not only may adifferent bioresorbable material be used to change the compressibilitycurve, but a combination of bioresorbable materials may be used.Composite bioresorbable materials may be formed to combinecompressibility characteristics, or the spacer(s) 46 may be formed oftwo or more distinct bioresorbable materials. The thickness of these twoor more materials may be selected to engineer the desiredcompressibility curve of the spacer 46 and thereby provide the mostbeneficial dynamization characteristics. The bone fastener 48 may bepositioned in the dynamization window 44 between a proximal spacerportion of one material and a distal spacer portion of a second materialso as to have tensile dynamization characteristics which differ fromcompressive dynamization characteristics. The spacers 46 in opposingwindows 44 may be of different sizes or formed of differentbioresorbable materials to control the bending dynamization relative tothe tensile and compressive dynamization. The present invention thusallows controlled dynamization across the fracture site 66, both forimproving fracture healing and for learning more about how dynamizationaffects the healing of the fracture.

The preferred PLA material does not include any active agents forrelease during bioresorption. Alternatively, the bioresorbable materialmay include an active agent as desired for release adjacent the fracturesite, such as an antibiotic or a growth factor.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For instance, while all of the theseattachments methods have been described with regard to the preferredbicortical attachment, unicortical attachment can also be used with ashorter bone fastener or by only advancing the bone fastener partiallythrough the nail 20.

What is claimed is:
 1. A bone support assembly for treatment of a bone,comprising: a bone support implant formed of a non-resorbable material,the bone support implant having a first window extending therethrough,the first window having a first window shape; and a first insert formedof a non-metal material separately from the bone support implant, thefirst insert having outer dimensions which correspond to the firstwindow shape, such that the first insert is insertable into the firstwindow and received by the first window to secure the first insertrelative to the bone support implant.
 2. The bone support assembly ofclaim 1, wherein the non-metal material of the first insert is abioresorbable material.
 3. The bone support assembly of claim 1, furthercomprising: a bone fastener having a length sufficient to extend throughthe first insert in attachment with a bone, the bone fastener having awidth small enough to be received in the insert and through the firstwindow, wherein the bone fastener is insertable through the insert whenthe insert is in the first window to attach the bone support implantrelative to a bone, with the direction of insertion of the bone fastenerin the insert being in the same direction as the insert is insertableinto the bone support implant.
 4. The bone support assembly of claim 3,wherein the outer dimensions of the first insert are sized to bereceived in the first window with a press fit.
 5. The bone supportassembly of claim 3, wherein the outer dimensions of the first insertare sized to be received in the first window with a snap fit.
 6. Thebone support assembly of claim 1, wherein the first window defines awindow axis, and wherein the first window comprises: a neck portionthrough which at least a major portion of the first insert may pass; anda mouth portion which extends from the neck portion and defines awidening opening relative to the window axis, the mouth portionextending at a different angle relative to the window axis than the neckportion.
 7. The bone support assembly of claim 1, wherein the firstinsert defines an insertion axis, and wherein the first insert has acontact surface which, during insertion into the first window, makessliding contact with the bone support implant, the contact surfacecomprising: a leading portion extending at a first angle relative to theinsertion axis; and a trailing portion extending at a second, differentangle relative to the insertion axis.
 8. The bone support assembly ofclaim 7, wherein the trailing portion defines a widening profilerelative to the insertion axis.
 9. The bone support assembly of claim 1,wherein the first insert defines a first insert axis defined in adirection of insertion into the bone support implant, the first inserthaving a contact surface which contacts the bone support implant duringor after insertion, the contact surface comprising: a first portionextending generally parallel to the first insert axis; and a secondportion extending at an angle relative to the first portion, the secondportion being wider than the first portion.
 10. The bone supportassembly of claim 1, wherein the first insert is packaged in a sealedcontainer for prolonged shelf-life and sealing the insert separate fromthe bone support implant, the sealed container substantially preventingthe first insert from contacting air.
 11. The bone support assembly ofclaim 10, wherein the sealed container comprises: a sealed inner pouch;and a sealed outer pouch surrounding the sealed inner pouch.
 12. Thebone support assembly of claim 10, wherein the inner pouch is formed ofa material from the group consisting of PET, foil and polyethylene, andwherein the outer pouch is formed of a material from the groupconsisting of paper, polyester and polyethylene.
 13. The bone supportassembly of claim 10, wherein the non-metal material is a bioresorbablematerial which has been sterilized by Cobalt 60 Gamma irradiation,Ethylene Oxide sterilization or gas plasma sterilization.
 14. The bonesupport assembly of claim 10, wherein the sealed container is flushedwith nitrogen upon sealing, such that the insert is retained in anitrogen environment for prolonged shelf-life.
 15. The bone supportassembly of claim 10, wherein the sealed container shields the insertfrom light.
 16. A bone support assembly for treatment of a bone,comprising: a bone support implant formed of a non-resorbable material,the bone support implant having a first window extending therethrough,the first window having a first window shape; and a first insert formedof a non-metal material separately from the bone support implant, thefirst insert having outer dimensions which correspond to the firstwindow shape, such that the first insert is insertable into the firstwindow and received by the first window to secure the first insertrelative to the bone support implant; and a second insert formed of anon-metal material separately from the bone support implant andseparately from the first insert, the second insert having outerdimensions which correspond to the first window shape, such that thesecond insert is insertable into the first window and received by thefirst window to secure the second insert relative to the bone supportimplant, the second insert having different mechanical or chemicaltreatment properties than the first insert.
 17. The bone supportassembly of claim 16, wherein the different mechanical or chemicaltreatment properties are selected from the group consisting of:different hardness, different rates of absorption, different activeagents and different amounts of active agents.
 18. A bone supportassembly for treatment of a bone, comprising: a bone support implantformed of a nonresorbable material, the bone support implant having atleast one window defined therein for exposure of a selected spacer; afirst spacer formed of a non-metal material, the first spacer beingsized such that it is receivable in the window in an exposed positionfor transverse fastening through the bone support implant and throughthe first spacer with a bone fastener; and a second spacer formedseparately from the first spacer, the second spacer being sized suchthat it is receivable in the window in an exposed position fortransverse fastening through the bone support implant and through thesecond spacer with a bone fastener, the second spacer having differentmechanical or chemical treatment properties than the first spacer. 19.The bone support assembly of claim 18, wherein the bone support implantis an intramedullary nail for insertion into the medullary canal of abone.
 20. The bone support assembly of claim 18, wherein the bonesupport implant is a bone plate for placement adjacent a bone.
 21. Thebone support assembly of claim 18, wherein an additional bone attachmenthole is defined in the bone support implant, the additional boneattachment hole being left open for insertion of a bone fastenertherethrough.
 22. The bone support assembly of claim 18, furthercomprising: a bone fastener having a length sufficient to extend throughthe first insert in attachment with a bone, the bone fastener having awidth small enough to be received in the insert and through the firstwindow.
 23. The bone support assembly of claim 18, wherein either thefirst insert or the second insert fills the first window prior toanchoring of a bone fastener transversely through the first insert. 24.The bone support assembly of claim 18, wherein the different mechanicalor chemical treatment properties are selected from the group consistingof: different hardness, different rates of absorption, different activeagents and different amounts of active agents.
 25. A bone supportassembly for treatment of a bone, comprising: a bone support implantformed of a non-resorbable material, the bone support implant having afirst attachment surface having a first attachment surface shape; and afirst insert formed of a non-metal material separately from the bonesupport implant, the first insert having outer dimensions whichcorrespond to the first attachment surface shape of the bone supportimplant, such that the first insert is attachable to the bone supportimplant with the outer dimensions of the first insert in a matingattachment with the first attachment surface to secure the first insertrelative to the bone support implant; and a second insert formed of anon-metal material separately from the bone support implant andseparately from the first insert, the second insert having outerdimensions which correspond to the first attachment surface shape, suchthat the second insert is attachable to the bone support implant withthe outer dimensions of the second insert in a mating attachment withthe first attachment surface to secure the second insert relative to thebone support implant, the second insert having different mechanical orchemical treatment properties than the first insert.
 26. The bonesupport assembly of claim 25, wherein the outer dimensions of each ofthe first insert and the second insert are sized to be attached to thefirst attachment surface with a press fit.
 27. The bone support assemblyof claim 25, wherein the outer dimensions of each of the first insertand the second insert are sized to be attached to the first attachmentsurface with a snap fit.
 28. The bone support assembly of claim 25,wherein the bone support implant is formed of metal and wherein thefirst and second inserts are formed of bioresorbable materials.